Double-reflector antenna with critical dimensioning to achieve minimum aperture blocking



6, 1965 P. w. HANNAN ETAL 3,

DOUBLE-REFLECTOR ANTENNA WITH CRITICAL DIMENSIONING TO ACHIEVE MINIMUM APERTURE BLOCKING Filed March 1. 1961 FIG. 2

FIG. 3

United States Patent 3,218,643 DOUBLE-REFLECTOR ANTENNA WITH CRITICAL DIMENSIONING TO ACHIEVE MINIMUM APER- TURE BLOCKING Peter W. Hannan, Northport, and Harold A. Wheeler, Great Neck, N.Y., assignors, by mesne assignments, to the United States of America as represented by the Secretary of the Army Filed Mar. 1, 1961, Ser. No. 92,504 3 Claims. (Cl. 343756) This invention relates to antennas having a feed cooperating with a subreflector which in turn cooperates with a main reflector, and more particularly to such antennas have minimum aperture blocking.

In the design of an optical telescope, the Cassegrain double-reflector system has often been utilized. Compared with the single-reflector type, it achieves a high magnification with a short focal length, and allows a convenient rear location for the observer. Recently, a number of microwave antennas have been developed which employ double-reflector systems similar to that of the Cassegrain telescope.

A Cassegrain telescope consists of two mirrors and an observing optical instrument. The primary mirror, which is a large concave mirror in the rear, collects the incoming light and reflects it toward the secondary mirror, which is a small convex mirror out in front. The secondary mirror then reflects the light back through a hole in the center of the primary mirror. When the incoming rays of light are parallel to the telescope axis, the final bundle of light rays is focused toward a point; at this location the observer places his eye or his camera.

In the basic microwave antenna derived from the Gassegrain telescope, the microwave reflectors, which will be called the main reflector and the subreflector, have surfaces similar in shape to those of the telescope. The microwave feed is a small antenna which, together with a transmitter or receiver, replaces the optical instrument of the telescope.

Analysis of the operation of the Cassegrain antenna system may be performed with the same semi-optical approximation commonly employed with an ordinary singlereflector antenna. Usually the feed is sufficiently small so that the wave radiated by the feed can be described by the far-field pattern of the feed before reaching the subreflector, and the wave incident on the subreflector appears to travel along the rays originating from a point centered on the feed. The subreflector, which must be large enough to intercept the useful portion of the feed radiation, ordinarily reflects this wave essentially according to ray optics. On reaching the main reflector, the Wave is again reflected according to ray optics, and, because of the geometry of the antenna elements, the rays emerge parallel and the wave front has the flat shape which is usually desired. The amplitude of the emergent wave across the aperture has a taper which is determined by the radiation pattern of the feed, modified by the additional tapering etfect of the antenna geometry. The far-field pattern of the antenna is a diffraction pattern whose characteristics depend on the amplitude taper of the emergent wave.

The geometry of the Cassegrain system is simple and well known. The classical Cassegrain geometry employs a parabolic contour for the main reflector and a hyperbolic contour for the subreflector. One of the two foci of the hyperbola is the real focal point of the system, and is located at the center of the feed; the other is a virtual focal point which is located at the focus of the parabola. As a result, all parts of wave originating at the real focal point, and then reflected from both surfaces, travel equal distances to a plane in front of the antenna. The invention is also applicable to variations of the classical Cassegrain system, including variations which resemble classical Gregorian telescopes. It is also applicable to other double-reflector systems having refleeting surfaces with contours quite different from those of the Cassegrain or Gregorian telescopes.

The principal limitation on the application of the historical Cassegrain system to microwave antennas is the blocking of the main aperture by the subreflector. This problem has not been serious with optical telescopes because the requirements on characteristics of the diffraction pattern have not been severe, and because, for the relatively short wave length of light, the size of the small reflector can be made very much less than that of the large reflector. With a microwave antenna, neither of these conditions ordinarily exists.

The presence of an opaque subreflector in the main aperture of the antenna creates a hole in the illumination which causes decreased gain and increased sidelobe levels. To analyze this effect, the resulting illumination may be resolved into two components, the original illumination plus a negative center, or hole. The resulting antenna pattern can be determined by adding together the two pattern components, the original patilerln plus a broad, low, negative pattern radiated by the Although the above method facilitates an exact calculation of the shadowing effect for any case, it is instructive to apply the method to a particular simple case which approximates many practical cases. If the main aperture is circular with diameter D and is assumed to have a completely tapered parabolic illumination, a small circular obstacle of the diameter D in the center of the aperture will create a hole pattern whose peak voltage E relative to the peak voltage E of the original pattern is:

E, D 2 si h) 1) where D is the diameter of the blocked portion of the aperture. This relative voltage is then subtracted from unity to yield the resultant relative peak voltage, and is added to the relative level of the first sidelobe to yield the resultant relative level.

The illumination hole is not the only effect created by the presence of an obstacle in the main aperture; the power which strikes the obstacle must also be accounted for. Usually this power re-radiates and contributes an additional component to the sidelobes. For a particular subreflector and antenna configuration, it is often a straight-forward process to estimate the amplitude pattern of this radiation. However the manner in which it combines with the original pattern is more complicated, and is likely to vary radically with a change of frequency. A further consideration of this effect is beyond the scope of this discussion and the effect will be neglected even though it may sometimes be important.

In order to determine the smallest amount of aperture blocking obtainable in a Cassegrain antenna having an opaque subreflector, it is necessary to consider those factors which influence the size of the subreflector. Essentially, the minimum size of the subreflector is determined by the directivity of the feed, and the distance between the feed and the subreflector. By making the feed more directive, or by decreasing its distance to the subreflector, the size of the subreflector may be reduced without incurring a loss caused by spillover of the feed radiation beyond the edge of the subreflector. However, a continuation of this process can eventually result in the feed itself creating a shadow in the main illumination which is greater than that created by the subreflector. There is some intermediate condition in which neither the subreflector nor the feed shadow predominates, and

which would yield the least amount of aperture blocking; this will be termed the minimum blocking condition for the purposes of this specification.

It is an object of this invention, therefore, to provide double-reflector antennas which avoid one or more disadvantages of the prior art.

It is an additional object of this invention to provide double-reflector antennas with minimum aperture blockmg.

In accordance with the invention a double-reflector antenna with minimum aperture blocking comprises a main reflector and subreflector means cooperating with the main reflector for reflecting a wave incident in an area having an equivalent diameter substantially equal to the square root of twice the product of the operating wave length times the focal length of the main reflector.

For a better understanding of the present invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accomapnying drawing, and its scope will be pointed out in the appeneded claims.

In the drawing:

In FIG. 1, the minimum blocking condition is shown. accordance with the invention, including notations helpful in explaining the invention;

FIG. 2 illustrates another double-reflector antenna in accordance with the invention, and

FIG. 3 includes two views of a polarization-changing screen utilized in the FIG. 2 antenna.

Referring to FIG. 1 of the drawing, there is shown an antenna having feed means 10, cooperating with a subreflector 11 which, in turn, cooperates with a main reflector 12. The feed means 10 act-s as a primary antenna, effecting the transition between a free-space electromagnetic wave and a guided electromagnetic wave and vice versa.

In FIG. 1, the minimum bolcking condition is shown. F represents the focal length of the main reflector 12 and F represents the distance between foci of the subreflector 11. Approximate equations can be written describing the basic relations between certain parameters of FIG. 1, as follows:

2i m g}; 2) a A=2. m; Ds a c and bmln= s where D min is the minimum blocking diameter, D is the equivalent diameter of the subreflector 11, Df is the equivalent diameter of the aperture of the feed 10, k is the ratio of the effective radiating feed aperture diameter to its equivalent diameter, 21 5 is the blocking angle subtended by the feed 10 as seen from the focal point of the main reflector 12 (the position of the focal point is indicated in FIG. 1 by the right end of F A is the blocking dimension of the feed 10 measured as determined by the blocking angle 2 and 2 is the blocking angle subtended at the aperture of feed 10 by the sureflector 11. Ordinarily k is slightly less than one since the diameter of the effective radiating feed aperture (the size of the electrical aperture) is always slightly less than the physical diameter of the feed because of the thickness of the metal of the feed; however, where a cluster of many feeds is employed to obtain a cluster of antenna beams, k can become quite small. Equivalent diameter means actual physical diameter where the component involved has a circular blocking area, or the equivalent eircualr diameter if a blocking area such as a square is involved. For an antenna in which only one of the two dimensions of the blocking area of a component involved is free to be altered to employ the minimum blocking condition, equivalent diameter applies to only this significant dimension. In the case of a. feed, the equivalent diam- 4 eter includes any blocking area associated with the feed.

By combining these equations, a relationship is obtained which specifies the geometry for the minimum blocking condition; it is as follows:

narrate (7 F 2 1 D,

This approximate analysis is a good approximation when the angle g3, and 5;, shown in FIG. 1 are small, and when the subreflector is much closer to the focus of the main reflector than it is to the feed. It also assumes that ray optics can describe the feed shadow; this is a good approximation when the feed is far from the subreflector. Within these limitations, minimum aperture blocking is obtained for a practical case in which there is essentially no spillover of the main lobe of the feed pattern past the edge of the subreflector. Although shown for the classical Cassegrain system, the above approximate analysis also applies for the classical Gregorian system, as well as other, less common double-reflector systems.

It can be seen that the minimum blocking condition is not limited to a particular set of antenna dimensions, but includes a series ranging from the case of a feed located near the vertex of the main reflector, and having a diameter about equal to that of the subreflector, to the case of a feed located far in front of the main reflector, and having a diameter much smaller than the subreflector. In the former case, the feed should be focused approximately toward the focal point of the main reflector in order that the illumination of the main aperture be characterized by a Fraunhofer diffraction pattern rather than a Fresnel pattern. In the latter case this is not necessary, but the feed must, of course, be excited by a length of transmission line and supported in its extended location; in an extreme form the latter case resembles a singlereflector antenna with a splash-plate feed, although the principle of operation is quite different.

The diameter of the aperture blocking for the minimum blocking condition is equal to an equivalent subreflector diameter substantially equal to the square root of twice the product of the operating wave length and the focal length of the main reflector. Operating wave length refers to the actual wave length of a single frequency system or the average or midband wave length over an operating frequency band width. This relationship is given by the following approximate equation:

b min or in the simple case where kwl b min\/ m where the limitations are the same as those mentioned previously. This equation also assumes that the total amount of aperture blocking is no greater than either of the two equal and coincident shadows. That is to say, the analysis by simple ray optics (rather than a more accurate analysis including wave diffraction, etc.) yields simple shadowing efiects which approximate the actual situation. Actually the blocking would be somewhat greater, particularly for the case of a small feed located close to the subreflector. In practice, the minimum condition can be substantially realized if the effective diameter ratio of the feed shadow to the subreflector is between /2 and l. The significant fact to note from Equation 8 is that the minimum blocking diameter can be computed before determining the feed size and location, these latter dimensions finally being related by Equation 7.

It is of interest to express (8) in some alternate approximate forms which more clearly illustrate the basic b min N D -10 D D 2k D -2k 241.,

where D is the equivalent diameter of the main reflector; 20 is the approximate half-power beam width of the antenna pattern in radians; and 2,, is the approximate included angle formed by the main reflector at its focus in radians. Actually, the first and second forms of (9) are almost exactly equal when the main aperture is circular and has a completely tapered parabolic illumination, and the second and third forms are equal when 2, is small. It is apparent from (9) that an antenna with a narrow beam width can have less relative aperture blocking than one with a wide beam width. This might be expected on the basis that the optical case, which has a very narrow beam width, has the capability for very small relative aperture blocking. Also apparent is the desirability of a small F/D ratio for the main reflector, and an eflicient feed aperture (k approaching one).

As an example, consider an antenna which is to have a pencil beam of one-degree half-power beam width, F /D =0.3, k=0.7, and which is to be designed for the minimum blocking condition. The second form of Equation 9 yields a value of about .012 for (D /D which may then be applied in Equation '1 to yield a value of about .024 for E /E The aperture blocking in this antenna would therefore reduce the gain by about db and would increase a 23 db sidelobe to about 2().5 db. (The 23 db figure is a value which is typical for the first sidelobe level when the illumination is tapered to about 11 db for maximum gain with a circular aperture.) This effect might be acceptable for some applications, but not for others. Thus, a one-degree beam width might :be considered as a rough boundary below which a doublereflector antenna designed for minimum aperture blocking would be very attractive. For larger beam widths, other types of antennas might be preferred.

The minimum blocking design will normally have either a larger feed than an antenna of ordinary design, or a feed located closer to the subreflector. As a result, the possibility of harmful reflections into the feed will be relatively greater. This defect can, in some applications, be eliminated by a polarization-changing device used in conjunction with the subreflector, as follows.

Referring now to FIG. 2, there is shown an antenna having a feed 15, cooperating with a subreflector 16 which, in turn, cooperates with a main reflector 17. In this case the feed happens to be located close to the main reflector 17 and the feed is substantially the same size as the subreflector 16. This antenna has a subreflector with an equivalent diameter substantially equal to the square root of twice the product of the operating wave length times the focal length of the main reflector times the ratio of physical diameter to effective diameter of the feed. The feed means are designed to have an equivalent diameter and location such as to cause blocking substantially equal to blocking caused by the subreflector means. The antenna of FIG. 2 further includes polarization-changing means in the form of screen 18. An example of such a screen is illustrated in FIG. 3. As shown in FIG. 3a, screen 18 is made up of a series of parallel metal wires such as 19, supported in a thin fiber glass skin 20. The skin 20 is supported and spaced from the metal surface 21 of the subreflector 16 by a dielectric honeycomb material 22 whose thickness is such that the wires are effectively separated from the metal surface by approximately of the effective operating wave length. The wire diameter and spacing are such that the normalized inductive susceptance equals approximately 2. A wave incident in direction 24 and having a linear polarization as indicated by arrow 25 in FIG. 3b, will be reflected with a polarization as shown by arrow 26. A screen such as 18 is effective in avoiding reflection back into the feed by changing the polarization of a wave reflected by the subreflector. For example, the screen 18 may be designed to change a vertically polarized wave from the feed to a horizontally polarized wave after reflection from the subreflector. The feed will then be substantially unaffected by the resulting horizontally polarized wave reflected toward it. A similar polarization-changing screen used in a different manner is described in more detail in applicants application, Serial No. 80,961, filed January 5, 1961, now Patent No. 3,161,879, and entitled Twistreflector, and also in applicants application, Serial No. 94,513, filed March 9, 1961, and entitled Double-Reflector Antenna With Polarization-Changing subreflector.

An antenna actually constructed in accordance with the invention, had the following dimensions. Only the more important dimensions are given, and all are normalized to wave length:

Using Equation 8, the D mm is calculated to be 15 /21 which is very close to the 16% actually obtained.

The polarization-changer dimensions were:

Wire 19 diameter .00461 Wire spacing O.18)\ Honeycomb 22 thickness 0.29). to 0.35) Skin 20 thickness 0.0161

While there have been described what are at present considered to be the preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention and it is, therefore, aimed to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. A double-reflector antenna with minimum aperture blocking comprising: a main reflector; subreflector means cooperating with said main reflector f-or reflecting a wave incident in an area having an equivalent diameter substantially equal to the square root of twice the product of the operating wave length times the focal length of the main reflector times the ratio of the equivalent diameter to the effective diameter of an associated feed; and polarization-changing means coupled to said subreflector means for changing the polarization of the wave reflected by said subreflector means.

2. A double-reflector antenna with minimum aperture blocking comprising: a main reflector; subreflector means cooperating with said main reflector for reflecting a wave incident in an area having an equivalent diameter substantially equal to the square root of twice the product of the operating wave length times the focal length of the main reflector; a polarization-changing screen coupled to said subreflector means; and feed means cooperating with said subreflector means for acting as a primary antenna with an equivalent diameter and location such as to cause blocking substantially equal to blocking caused by the subreflector means.

3. A double-reflector antenna with minimum aperture blocking comprising: a main reflector; subreflector means cooperating with said main reflector for reflecting a wave incident in an area having an equivalent diameter substantially equal to the square root of twice the product of the operating wave length times the focal length of the main reflector times the ratio of the equivalent diameter to the effective diameter of an associated feed; a polarization-changing screen coupled to said subreflector means; and feed means cooperating with said subreflector means for acting as a primary antenna with an equitor means.

References Cited by the Examiner UNITED STATES PATENTS LuX 343838 Boerner 343838 8 6/1960 Mattingly 343-756 X 5/1964 Privett et a1. 343782 FOREIGN PATENTS 1/ 1953 Germany. 7/1945 Great Britain. 3/1960 Sweden.

HERMAN KARL SAALBACH, Primary Examiner.

Cutler 343781 10 GEORGE N. WESTBY, Examiner. 

1. A DOUBLE-REFLECTOR ANTENNA WITH MINIMUM APERTURE BLOCKING COMPRISING: A MAIN REFLECTOR; SUBREFLECTOR MEANS COOPERATING WITH SAID MAIN REFLECTOR FOR REFLECTING A WAVE INCIDENT N AN AREA HAVING AN EQUIVALENT DIAMETER SUBSTANTIALLY EQUAL TO THE SQUARE ROOT OF TWICE THE PRODUCT OF THE OPERATING WAVE LENGTH TIMES THE FOCAL LENGTH OF THE MAIN REFLECTOR TIMES THE RATIO OF THE EQUIVALENT DIAMETER TO THE EFFECTIVE DIAMETER OF AN ASSOCIATED FEED; AND POLARIZATION-CHANGING MEANS COUPLED TO SAID SUBREFLECTOR MEANS FOR CHANGING THE POLARIZATION OF THE WAVE REFLECTED BY SAID SUBREFLECTOR MEANS. 